{"schema":"pubphys.bundle/1","record":{"author":{"account_ref":null,"orcid":null},"builds_on":[],"content_schema":"pubphys.content.revision/1","content_sha256":"a8e5a37a9d3df09b3c96689103e8596854e06e57c31574ea11718268a05d9f75","created":"2026-10-15T23:43:22Z","files":[],"origin":{"assisted_by":[],"kind":"seed"},"parents":["276b45bf115bdc22900108a066b9e0809155868eb17d0df3eeb450dc17f6e047","ce59fb6cd812fc05906d0c60a71c75ea22bc686bcf38e7c3ef7010834d98ee7b"],"salt":"4a910b74621816b75d0aee42b8c30b29c9ece0f393475f663ba0a96b159960bb","schema":"pubphys.record/2","site":"pubphys.com","target":null,"type":"revision"},"content":{"answer_type":"value","assisted_by":[],"external_id":"beams.beam-brightness.photoinjector-brightness-limit","kind":"well-posed","literature_status":"open","n":"2","parents":[],"plain":"Theory gives a maximum brightness set by the field at the cathode and the electrons' sideways energy; real guns fall short by an amount that is not fully explained.","posed_since":"2009","precise":"For short-pulse photoinjectors the maximal 4D normalized brightness of a pancake-shaped bunch is bounded by the cathode field $E_c$ and MTE (bound derived in 2009). Determine whether measured 4D or 5D brightness of DC, normal-conducting RF and superconducting RF guns can come within a factor 2 of this bound at bunch populations of $1\\mathrm{e}8 \\text{ to } 1\\mathrm{e}9\\,\\mathrm{electrons}$, and which effects (image charge, nonuniform emission, imperfect emittance compensation, cathode roughness) cause the observed gap.","problem_ref":"276b45bf115bdc22900108a066b9e0809155868eb17d0df3eeb450dc17f6e047","references":"","settled_by":"Measured slice brightness of one gun at several $E_{c}$ and MTE values compared with the bound and with a start-to-end model that accounts for the gap.","status_note":"","title":"Can photoinjectors reach the space-charge-limited brightness 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